1. Field of the Invention
The present invention generally relates to active fixation elements, and in particular such fixation elements for usage in implantable medical leads.
2. Description of the Prior Art
Body implantable electrical leads form the electrical connection between an implantable medical device (IMD), such as cardiac pacemaker or cardiac defibrillator, and body tissue, such as the heart, which is to be electrically stimulated. As is well known, the leads connecting the IMD with the tissue may be used for pacing/defibrillation and for sensing electrical signals produced by the tissue.
The implantable leads of today and in particular cardiogenic implantable leads can generally be divided into two classes depending on the tissue anchoring arrangement of the leads. Firstly, so-called passive fixation leads comprise radially protruding elements in the distal lead ends. These elements can become embedded in the trabecular network inside the heart and thereby provide an anchoring of the lead to the heart tissue. Examples of such protruding elements include collars, tines and fines. Passive leads are generally characterized by low chronic capture threshold and high impedance.
The other class of leads includes so-called active fixation leads. Such a lead typically comprises, in its distal end, a helical fixation element that can be screwed into the endocardium and myocardium to provide the necessary lead-to-tissue anchoring. Generally, active leads have superior ability to fixate without the need for any trabecular network.
When a helical fixation element penetrates the myocardium, it causes a local injury and bleeding typically occurs together with an inflammatory response. Blood will leak into the space between the endocardium and the myocardium, forming a blood pocket. Taken together these effects result in the formation of a blood clot and a fibrin network around the anchored fixation element. If the fixation element is also employed as one of the pacing and/or sensing electrodes of the lead, the formed blood clot will lead to higher capture thresholds and lower sensibility of the helical electrode.
The traditional approach of mitigating these problems has been to include a steroid plug or some other steroid source at the distal lead end. The steroid reduces the inflammatory response following the tissue anchoring and thereby decreases the amount of non-stimulable tissue (fibrin) around the fixation element.